Many ion trap devices currently used are so-called three dimensional quadrupole ion trap devices, which are composed of a ring electrode and a pair of end cap electrodes placed opposite to each other with the ring electrode therebetween, each electrode having an inner surface shaped as a hyperboloid of revolution. Normally, a radio frequency (RF) voltage is applied to the ring electrode to produce a quadrupole electric field in the space (ion trapping space) surrounded by the electrodes for trapping ions in the ion trapping space. The kinetic state of the ions is different depending on their mass to charge ratios, which is used to discriminate or dissociate ions.
Such an ion trap device may be used as a mass spectrometer by itself, or it may be used as an ion source for a subsequent ion analyzer. For example, “A Marriage Made in MS” by M. G Qian and D. M. Lubman, Analytical Chemistry, vol. 67 (1995), No. 7, p. 234A, discloses a multi-stage mass spectrometer in which a three-dimensional quadrupole ion trap is placed before a time-of-flight mass spectrometer (TOFMS). In the multi-stage mass spectrometer, a multi-stage mass analysis is first made in the ion trap, and then the ions are injected into the high-resolution TOFMS to obtain a mass spectrum.
Such a construction of mass spectrometer that an ion trap device is provided before another mass analyzer and ion analyses are successively performed has generated new types of mass analyzers. However, a problem in this structure is that an operation parameter or parameters applied in the ion trap device when ions are transferred from the ion trap to the subsequent mass analyzer may affect the performances of the mass analyzer. For example, the initial kinetic energy of ions transferred from the ion trap to the mass analyzer may change depending on the ion-trapping RF voltage applied to the ring electrode of the ion trap device, and an ion-ejecting high DC voltage or voltages applied to the end cap electrodes may generate a voltage pulse (voltage spike) in the ring electrode, which also changes the initial kinetic energy of the ejected ions.
In a mass spectrometer using a quadrupole ion trap device and a TOFMS, for example, ions trapped in the ion trap keep moving due to the RF voltage applied to the ring electrode. When the ions are to be ejected from the ion trap, appropriate voltages are applied to the end cap electrodes respectively to drive and accelerate the ions toward the subsequent TOFMS. Specifically, as described in U.S. Pat. No. 6,380,666, when ions are ejected, the voltage to the ring electrode is dropped to zero, and a +6 kV DC voltage is applied to the introduction end cap electrode (through which ions enter the ion trap) and −10 kV DC voltage is applied to the extraction end cap electrode. This drives the ions (cations) in the ion trap device toward the extraction end cap electrode, and the ions are ejected through a hole at the center of the extraction end cap electrode to the TOFMS.
In U.S. Pat. No. 6,483,244, a sophisticated method is disclosed for reducing the RF voltage of the ring electrode to zero before ions are ejected to minimize the influence of the ion-trapping RF voltage on the initial kinetic energy of the ions.